I didn't answer to the poll, but most interviews I've been on lasted around 10-15 minutes long. I have gotten some of my jobs from interviews that lasted that long. It all depends on the job, how you presented yourself while on the interview, how many others were being interviewed other than yourself! If you don't hear anything from them in a few days, go back and ask them about the status of your application, and let them know you were already interviewed.

15 minutes seems like an average time for a basic job but the length of time doesn't mean you'll get the job. I've had one job where the employer was so desperate, he didn't care for having an interview, while a different employer asked 5 quick questions, interrupted me before I could ever finish and in both, I got the jobs. They weren't anything amazing, mostly grunt work but for higher-paying jobs, 15 minutes probably is an average or even a minimum amount of time.

I can rip you off, and steal all your cash, suckerpunch you in the face, stand back and laugh. Leave you stranded as fast as a heart-attack.
- Danko Jones (I Think Bad Thoughts)